In general, the sensing technologies employed in domestic grade smoke detectors are unable to reliably discriminate between smoke or aerosols from genuine fire hazards and those arising from non-hazardous sources such as cooling/burning food, steam etc. Consequently, the occurrence of “false” or nuisance alarms has long been recognised as a limitation of smoke detection in domestic environments. This generally precludes the use of smoke detectors in or near to kitchens; for instance.
Nuisance alarms not only cause annoyance to the consumer, it is quite common for domestic protection systems in the US to be linked directly to the local fire service. False alarms can therefore incur significant costs to homeowners (and businesses), as well as being a drain on fire service resources.
More advanced smoke detection methodologies can introduce an element of nuisance alarm rejection based on intelligent interpretation of multiple parameter measurements. However the cost premium associated with more complex sensing arrangements is not compatible with the low-cost domestic market and, although such enhancements would improve detector integrity, the problem of false alarms would not be adequately resolved for reliable domestic use.
Since detection-level false alarm avoidance is difficult, the more common option is to provide features that override nuisance alarms to limit the disturbance and inconvenience caused. Many basic smoke detector models have no means by which to silence a false alarm. Higher specified models however, often incorporate a “hush” (or otherwise named) feature which essentially allows the end user to temporarily “de-sensitise” a detector, causing alarms due to low-level smoke to be silenced (see, for example, U.S. Pat. No. 4,814,748). It is normal for the “hush” mode to be activated manually via a button on the detector body. In the “hush” mode, smoke detectors remain sensitive to smoke, but operate with a higher alarm threshold. Therefore, if more significant smoke levels are detected whilst the detector is temporarily “de-sensitised”, the device will still enter alarm mode. After a pre-defined time period the alarm thresholds are reset to normal. The precise protocol used in such features is stringently governed by the relevant standards.
WO02063216 discloses a system which uses smoke and/or gas detection equipment to monitor the emissions from cooking food in order to control the cooking appliance. The system is incorporated in the cooking device (examples given include toasters, toaster ovens, bread machines and microwave ovens) and actively samples smoke/gas from the main oven/appliance interior into a separate sensing chamber. The measured parameters are fed back to the cooking device and are used to control the appliance settings, e.g. reduce the cooking temperature or vary the cooking time, until the smoke and/or gas levels return to “normal”.
GB2275556 discloses a system which is intended to add protection to appliances that use heating elements and are prone to catching fire (washing machines, dryers etc). Essentially, smoke detection apparatus is installed within the body of appliances, so that localised smoke generated within the appliance can be detected and used to alert external smoke detectors to the hazard, i.e. before smoke emerges from within the appliance.